MECHANISM OF EMPHYSEMA. 593 
that the microscope and other modern means of investigation, 
which have done so much for morbid anatomy, have scarcely 
availed here to augment our knowledge ; having only succeeded 
in demonstrating more clearly the fact, known to Laennec, of 
the gradual breaking up of the vesicular septa, and the oblitera- 
tion of their capillary network. 
Emphysema, therefore, is an abnormal distention of the pul- 
monary tissue with air. In its earliest stages, whether inter- 
lobular or vesicular, or, as frequently happens, both combined, 
nothing can be more certain than that it is essentially a mecha- 
nical lesion : in fact, the distention of the air-cells, giving the 
peculiar cushion -like and pale appearance to the lung, can be 
exactly imitated by inflating it with undue force artificially. 
Moreover; the whole of the subsequent structural changes im- 
plied in the gradual removal of the septa and obliteration of the 
capillaries, are readily explained by the mechanical effects of 
distention. Upon this subject M. Poiseuille, to whom we owe 
so many interesting facts in mechanical physiology, has a very 
beautiful experiment. 
An instrument being adapted to the pulmonary artery of an 
animal, by which a given quantity of liquid was propelled with 
a given force through the capillaries of the lung, he found that 
this was effected, in the normal condition, in 29 seconds. M. 
Poiseuille now inflated the lungs so as exactly to fill the cavity 
of the chest; the time was still 29 seconds. On distending 
the lungs, however, farther, so as to produce the appearance of 
a partial emphysema, the time required for the passage of the 
fluid became lengthened to 62 seconds ; when the emphyse- 
matous appearance was increased, 95 seconds; when it per- 
vaded the whole lung in consequence of excessive distention, 
129 seconds were required, and the fluid returned from the 
pulmonary veins mixed with some bubbles of air. From these 
results, it is evident that whenever the air-cells are distended 
beyond the amount required or possible in the healthy condi- 
tion, the flow of blood through the ultimate capillaries of the 
lung must be retarded or obstructed ; a condition not only cor- 
responding with the appearances observed in emphysema, but 
readily accounting for the structural changes, the absorption of 
the walls of the air-cells, and the tension and obliteration of 
vessels observed in the latter stages of the disease. 
It is, therefore, nearly certain that the source of emphysema 
is to be sought in a derangement of the mechanism of respira- 
tion, and not in any previously morbid condition of the affected 
part. Every thing denotes that the emphysematous parts of a 
lung are usually free from all diseased changes, with the ex- 
ception of those which are the result of inordinate distention. 
